QR codes for book layout

It's all about resolution. The codes must be large enough, and sharp enough, for a reading device to be able to read them without hesitation and ideally without relying too much on the error-correction factors.

For regular black-only interior printing on uncoated paper, I'd make sure the codes are at least 2mm across each grid element (square). 1mm is about the reasonable limit for a very sharp image and most trade printing.

Format can be any of the standard types. Vector (AI or PDF) would be best. Lossless PNG is good. Low-compression JPEG with no anti-aliasing is usually acceptable. But it's all about providing a good source file that is (ideally) an even multiple of the print resolution and can be scaled and placed with no anti-aliasing or sub-pixel rescaling. There should be no — none, nada, zip — aliasing, shading or pixelation of the grid squares, even under magnification.

It helps to keep the encoded information short, just a bare URL with as short an addressing 'tail' as possible. That increases the grid size and allows you to make the code a little smaller and still preserve quick readability.

And then there's contrast; nothing beats black-on-white or the reverse. Any use of colors, shading, tints etc. should be tested and used judiciously, with as high a contrast ratio as can be managed.